Actually both the wards and an AH are pretty much on the same ground for undercutting. Yes market wards show the exact price whereas the AH was a bit of a mystery, however at the same time people undercutted since the lower price always took priority on what sold first.
This is true, which is why I said there is downsides, but I found that Market Wards undercutting was worse than Auction House which is why I like the latter system better. With AH its a bit harder to keep a price up because of the history. But a lot of the time, people will undercut by 1 gil, 10gil, 100g, 1000g. Just NOT AS OFTEN by 20k or whatever people do with the Market Wards.